Switch mode power supplies are widely used in power conversion applications. A Buck converter is one of the well-known switch mode power supplies. The buck converter which comprises a high side power device 11, a low side power device 12, an inductor 13 and a capacitor 14 coupled as shown in FIG. 1 steps down an input voltage (VIN) to a lower output voltage (VOUT). Typically, the high side power device and the low side power device are DMOS devices due to the good performance of the DMOS devices.
In high voltage applications (e.g., the input voltage VIN may be higher than 100V), the size of a high side DMOS tends to be bigger than that of a low side DMOS. That is because the high side DMOS needs an N-buried layer (NBL) to isolate the body from the substrate. Then the breakdown voltage of the high side DMOS mainly relies on the doping of the drift region. While the low side DMOS may use RESURF action from the N-type well to the substrate junction to maintain the high breakdown voltage with no N-buried layer.
But with the development of the semiconductor industry, higher breakdown voltages and lower Ron*A (wherein A represents the area of the device) are both required.